Slurry phase polymerisation process

ABSTRACT

A process comprising polymerizing an olefin monomer optionally together with an olefin comonomer in the presence of a polymerization catalyst in a diluent in a loop reactor which comprises at least 2 horizontal sections and at least 2 vertical sections to produce a slurry comprising solid particulate olefin polymer and the diluent wherein the Froude number in at least 20% of the length of the vertical sections of the reactor loop is less than 85% of the Froude number in at least 20% of the length of the horizontal sections of the loop is disclosed.

This application is a continuation of application Ser. No. 11/667,018,filed Jun. 9, 2008, now U.S. Pat. No. 7,572,866, the entire content ofwhich is hereby incorporated by reference in this application.

BACKGROUND OF THE INVENTION

The present invention is concerned with olefin polymerisation in slurryor suspension phase loop reactors.

Slurry phase polymerisation of olefins is well known wherein an olefinmonomer and optionally olefin comonomer are polymerised in the presenceof a catalyst in a diluent in which the solid polymer product issuspended and transported.

This invention is specifically related to polymerisation in a loopreactor where the slurry is circulated in the reactor typically by meansof a pump or agitator. Liquid full loop reactors are particularly wellknown in the art and are described for example in U.S. Pat. Nos.3,152,872, 3,242,150 and 4,613,484.

Polymerisation is typically carried out at temperatures in the range50-125° C. and at pressures in the range 1-100 bara. The catalyst usedcan be any catalyst typically used for olefin polymerisation such aschromium oxide, Ziegler-Natta or metallocene-type catalysts. The productslurry comprising polymer, and diluent, and in most cases catalyst,olefin monomer and comonomer can be discharged intermittently orcontinuously, optionally using concentrating devices such ashydrocyclones or settling legs to minimise the quantity of fluidswithdrawn with the polymer.

The loop reactor is of a continuous tubular construction comprising atleast two, for example four, vertical sections and at least two, forexample four, horizontal sections. The heat of polymerisation istypically removed using indirect exchange with a cooling medium,preferably water, in jackets surrounding at least part of the tubularreaction loop. The volume of the loop reactor can vary but is typicallyin the range 20 to 120 m³the loop reactors of the present invention areof this generic type.

Maximum commercial scale plant capacities have increased steadily overthe years. Growing operating experience over the last few decades hasled to operation of increasingly high slurry and monomer concentrationsin reaction loops, the increase in slurry concentrations has typicallybeen achieved with increased circulation velocities achieved for exampleby higher reactor circulation pump head or multiple circulation pumps asillustrated by EP 432555 and EP 891990. The increased velocity and headrequirement has led to increasing energy consumption as slurryconcentrations increase. Despite increased operating experience thevolume of individual polymerisation reactors has also needed to beincreased to accommodate the desired production capacity. Constructionand commissioning of new commercial plants is very expensive andtherefore new designs seek to achieve any required scale-up in capacitywhilst changing parameters that present minimum risk to the successfuloperation of the new unit. Typically reactor loop volume has beenincreased by adding legs and/or length to existing reactor loops or evenby linking two existing loops together whilst maintaining the reactorloop internal diameter at about 24″ (600 millimeters) or below. Theincrease in reaction loop volume by increasing length at a fixeddiameter leads to steadily increasing absolute (and even specific) looppressure drops (and therefore power consumption).

Increasing the diameter of commercial scale reactors to increase reactorvolume has been seen as giving greater scale-up risk than thatassociated with increasing length. The increased risk has beenassociated with concerns over maintenance of good thermal, compositionaland particle distribution across the reactor cross-section withoutexcessively increasing turbulence (e.g. circulation velocity) andassociated pressure drop/power in the polymerisation loop. Inadequatecross-sectional distribution could lead to increased fouling, reducedheat transfer and reduced polymer productivity and homogeneity.

In addition, reactors are typically designed and constructed with aconstant internal diameter around the entire loop, except for examplewhere fittings, such as the circulation pumps, dictate a different(larger or smaller) diameter at a specific location for a particularreason. There would have been an expectation that varying the internaldiameter between for example the vertical and horizontal sections wouldlead to fouling problems. We have found that this is not the case.

DESCRIPTION OF THE INVENTION

In accordance with the present invention there is provided a processcomprising polymerising an olefin monomer optionally together with anolefin comonomer in the presence of a polymerisation catalyst in adiluent in a loop reactor which comprises at least 2 horizontal sectionsand at least 2 vertical sections to produce a slurry comprising solidparticulate olefin polymer and the diluent wherein the Froude number inat least 20% of the length of the vertical sections of the reactor loopis less than 85% of the Froude number in at least 20% of the length ofthe horizontal sections of the loop.

Advantages of the invention are that the residence time of a givenlength of reactor is increased while simultaneously minimising anyincrease in risk of reactor fouling. The invention enables design andoperation of vertical slurry loop reactors with reduced total andspecific energy consumption.

This invention relates to a method and apparatus for continuouspolymerization of olefins, preferably alpha mono olefins in a verticalelongated tubular loop reaction zone. The olefin(s) is continuouslyadded to, and contacted with, a catalyst in a hydrocarbon diluent. Themonomer(s) polymerise to form a slurry of solid particulate polymersuspended in the polymerisation medium or diluent. In particular, theinvention is related to a process where the Froude number varies aroundthe loop

The Froude number is a dimensionless parameter indicative of the balancebetween the suspension and settling tendencies of particles in a slurry.It provides a relative measure of the momentum transfer process to thepipe wall from particles compared to the fluid. Lower values of theFroude number indicate stronger particle-wall (relative to fluid-wall)interactions. The Froude number (Fr) is defined as v²/(g(s−1)D) where vis the velocity of the slurry, g is the gravitational constant, s is thespecific gravity of the solid and D is the pipe diameter The specificgravity of the solid polymer which is the ratio of the density of thepolymer to the density of water is based on the annealed density of thedegassed polymer after being substantially devolatilised and immediatelyprior to any extrusion as measured using method ISO1183A.

The Froude number in at least 20% of the length of the vertical sectionsof the reactor loop is less than 85% of the Froude number in at least20% of the length of the horizontal sections of the loop

The average Froude number in the loop will preferably be maintained ator below 20, for example in the range 20 to 1 preferably in the range 15to 2, more preferably in the range 10 to 3.

Typically, in the slurry polymerisation process of polyethylene, theslurry in the reactor will comprise the particulate polymer, thehydrocarbon diluent(s), (co) monomer(s), catalyst, chain terminatorssuch as hydrogen and other reactor additives. In particular the slurrywill comprise 20-75, preferably 30-70 weight percent based on the totalweight of the slurry of particulate polymer and 80-25, preferably 70-30weight percent based on the total weight of the slurry of suspendingfluid, where the suspending medium is the sum of all the fluidcomponents in the reactor and will comprise the diluent, olefin monomerand any additives; the diluent can be an inert diluent or it can be areactive diluent in particular a liquid olefin monomer where theprincipal diluent is an inert diluent the olefin monomer will typicallycomprise 2-20, preferably 4-10 weight percent of the slurry.

The solids concentration in the slurry in the reactor will typically beabove 20 volume %, preferably about 30 volume %, for example 20-40volume %, preferably 25-35 volume % where volume % is [(total volume ofthe slurry−volume of the suspending medium)/(total volume of theslurry)]×100. The solids concentration measured as weight percentagewhich is equivalent to that measured as volume percentage will varyaccording to the polymer produced but more particularly according to thediluent used. Where the polymer produced is polyethylene and the diluentis an alkane for example isobutane it is preferred that the solidsconcentration is above 40 weight % for example in the range 40-60,preferably 45%-55 weight % based on the total weight of the slurry.

It is a particular feature of the present invention that operation ofthe slurry phase polymerisation at variable, preferably low Froudenumbers enables the reactor to be run at high solids loading. Apreferred embodiment of the present invention is a process comprisingpolymerising in a loop reactor an olefin monomer, in particularethylene, optionally together with an olefin comonomer in the presenceof a polymerisation catalyst in a diluent, particularly isobutane, toproduce a slurry comprising solid particulate olefin polymer and thediluent wherein the Froude number in at least 20% of the length of thevertical sections of the reactor loop is less than 85% of the Froudenumber in at least 20% of the length of the horizontal sections of theloop

The present invention may be carried out in reactors having an averageinternal diameter of over 300 millimeters. The present invention ispreferably carried out in larger diameter reactors than areconventionally used in slurry polymerisation. For example, reactorshaving average internal diameters over 500 millimeters, in particularover 600, for example between 600 and 750 millimeters are preferablyused. A further advantage of this invention is therefore that highslurry concentrations at relatively low circulation velocities and/orrelatively high reactor loop diameters can be achieved. A furtherembodiment of the present invention is a process comprising polymerisingin a loop reactor an olefin monomer optionally together with an olefincomonomer in the presence of a polymerisation catalyst in a diluent toproduce a slurry comprising solid particulate olefin polymer and thediluent wherein the Froude number in at least 20% of the length of thevertical sections of the reactor loop is less than 85% of the Froudenumber in at least 20% of the length of the horizontal sections of theloop and the average internal diameter of the reactor is in the range600-750 millimeters.

The average internal diameters of the vertical sections can be the same,greater or less than, preferably greater than the average internaldiameter of the horizontal sections. Typically the horizontal sectionswill have an average internal diameter in the range 500-700 millimetersfor example in the range 600 to 650 millimeters. The vertical sectionswill typically have an average internal diameter in the range 600-900,for example 650-750 millimeters. The average internal diameter of eachof the horizontal sections and each of the vertical sections can be thesame or different. The internal diameter can remain the same or varyalong, a single horizontal or vertical section, preferably it remainsthe same. The average internal diameter of the vertical sections can beup to 90% for example 5-90% such as 5-50% in particular 10-30% greaterthan the average internal diameter of the horizontal sections.

Vertical and horizontal shall be taken to mean substantially verticaland substantially horizontal respectively which for example will be notgreater than 10 degrees preferably not greater than 5 degrees, from thegeometric vertical and geometric horizontal respectively.

The polymerisation mixture or slurry (as defined above) is pumped aroundthe relatively smooth-path endless loop reaction system at fluidvelocities sufficient to (i) maintain the polymer in suspension in theslurry and (ii) to maintain acceptable cross-sectional concentration andsolids loading gradients.

It has been found that vertical sections of reactor loops may beoperated with Froude numbers in the vertical sections of the reactorthat are significantly lower than the minimum required in the horizontalsections to maintain reliable reactor operations. Whilst the momentumtransfer process to the pipe wall of particles relative to fluid isclearly significantly reduced in this case it has been found thatacceptable heat transfer and heat transfer coefficients may still bemaintained without affecting plant reliability.

The Froude number in the vertical sections is preferably maintained atbetween 15% and 85% of the minimum Froude number used in the horizontalsections. The Froude number in the vertical sections with upwardcirculation is preferably maintained at between 30% and 85% of theminimum Froude number used in the horizontal sections. Much lowerrelative Froude numbers are possible in the vertical sections withdownward circulation. The Froude number in the vertical sections withdownward circulation is preferably maintained at between 15% and 70% ofthe minimum Froude number used in the horizontal sections.

In one embodiment of the invention the Froude number in the horizontalsections of the loop is maintained below 30, preferably less than 20,most preferably less than 10 and the Froude number in the verticalsections is maintained below 20, preferably less than 10, mostpreferably below 5.

In a preferred embodiment of the invention the Froude number in at least20% of the length of the vertical sections of the reactor loop withdownward circulation is maintained at less than 85% of the Froude numberpresent in at least 20% of the length of the vertical sections of theloop with upward circulation.

In an alternative embodiment of the invention at least 20% of the lengthof the vertical sections of the reactor loop have an internalcross-sectional area at least 5% greater than the largest internalcross-sectional area that covers at least 20% of the length of thehorizontal sections of the loop.

In a further embodiment of the invention the Froude number in at least20% of the length of the vertical sections of the reactor loop withdownward circulation is maintained at less than 85% of the Froude numberpresent in at least 20% of the length of the vertical sections of theloop with upward circulation.

Preferably the horizontal sections consist of no more than 20% of thereactor length and/or contribute no more than 20% of the reactor volume.

In a particular embodiment the downward flowing section is sized tomaximise reactor volume and catalyst productivity, even though the heattransfer coefficient in this case may not be as high as normallyexpected. In this case the circulation rate in the downward flowingvertical sections can even be less than the minimum settling velocity ofthe particles in the reactor. In a preferred embodiment the Froudenumber in the downward flowing vertical sections is maintained atbetween 1 and 5, preferably between 1 and 3. Although the total reactorlength in this case may be higher than would otherwise be needed frompurely heat transfer considerations, it has been found that this designmethodology leads to a new optimum design point that balances catalystproductivity and pump power.

As reactor diameter for a fixed reactor volume increases so theavailable heat transfer area decreases. A further advantage of thisinvention is that it has been found that high slurry concentrations canbe tolerated in large diameter reactors at relatively low circulationvelocities, as circulation velocities decrease so too does heat transfercoefficient (all other things being equal). When employing the fulladvantages of this invention any reactor design is more likely to beheat transfer limited than catalyst productivity or space time yieldlimited, this means that lower catalyst residuals can be achieved thanin an equivalent reactor designed by prior art methods.

It has been found that reactors can be designed and operated at specificpressure drop both per unit reactor length and per mass of polymer andtotal pressure drop for the loop less than that taught as being requiredat high solids loadings in the prior art. This invention permits totalloop pressure drops of less than 1.3 bar, typically less than 1 bar,preferably less than 0.8 bar even for polymer production rates of above25, even above 45 tonnes per hour. It is possible to employ one pump ormore than one pump in the loop preferably on one or more horizontalsections; these can be located on the same horizontal section or ondifferent sections. The pump or pumps can be of the same diameter orlarger or smaller diameter preferably of the same diameter as theinternal diameter of the section of the reactor where the pump or pumpsare located. It is preferable to employ a single pump and it is afeature of the present invention that requirements for number and powerof pump(s) is less onerous than for conventional processes.

Reactor size is typically over 20 m³ in particular over 50 m³ forexample 75-150 m³ preferably in the range 100-125 m³

The ability to operate at low Froude numbers in the vertical sectionsenables larger reactor diameters to be considered and enables reactorvolumes, for example of greater than 80 m³ to be built with reactorlength to average internal diameter ratios of less than 500, preferablyless than 300 for example less than 250. Reduction in reactor length toaverage internal diameter ratio minimises compositional gradients aroundthe reaction loop and enables production rates of greater than 25 te/hrfor example greater than 40 te/hr per reactor to be achieved with only asingle point of reagent introduction around the reaction loop.Alternatively it is possible to have multiple inlets into the loopreactor for reactants (e.g. olefins), catalyst, or other additives.

In a preferred embodiment of the invention the loop is designed so thatthe Froude number in any vertical section of the loop within 5 pipediameters, preferably 10, most preferably 15 pipe diameters upstream ofa horizontal section of the loop is maintained at no less than 90%,preferably about 100%, of the Froude number in that horizontal sectionof pipe. This is to ensure that the fluid has reached approximately thesame conditions as in the horizontal section before entering thehorizontal section.

The pressure employed in the loop is sufficient to maintain the reactionsystem ‘liquid full’ i.e. the diluent and reagents (i.e. monomers andchain terminators) substantially in a liquid phase, normally pressuresused are between 1-100 bara, preferably between 30 to 50 bara. Inethylene polymerization the ethylene partial pressure is most oftenchosen from 0.1 to 5 MPa, preferably from 0.2 to 2 MPa, moreparticularly from 0.4 to 1.5 MPa. The temperatures selected are suchthat substantially all of the polymer produced is essentially (i) innon-tacky and non-agglomerative solid particulate form and (ii)insoluble in the diluent. The polymerization temperature depends on thehydrocarbon diluent chosen and the polymer being produced. In ethylenepolymerisation it is generally below 130 C, typically between 50 and 125C, preferably between 75 and 115 C. For example in ethylenepolymerisation in isobutane diluent, the pressure employed in the loopis preferably in the range 30-50 bara, the ethylene partial pressure ispreferably in the range 0.2-2 MPa and the polymerisation temperature isin the range 75-115 C. The space time yield which is production rate ofpolymer per unit of loop reactor volume for the process of the presentinvention is in the range 0.1-0.4 preferably 0.2-0.35 ton/hour/m³.

The process according to the invention applies to the preparation ofcompositions containing olefin (preferably ethylene) polymers which cancomprise one or a number of olefin homo-polymers and/or one or a numberof copolymers. The process according to the invention is particularlysuited to the manufacture of ethylene and propylene polymers. Ethylenecopolymers typically comprise an alpha-olefin in a variable amount whichcan reach 12% by weight, preferably from 0.5 to 6% by weight, forexample approximately 1% by weight.

The alpha mono-olefin monomers generally employed in such reactions areone or more 1-olefins having up to 8 carbon atoms per molecule and nobranching nearer the double bond than the 4-position. Typical examplesinclude ethylene, propylene, butene-1, pentene-1, and octene-1, andmixtures such as ethylene and butene-1 or ethylene and hexene-1.Butene-1, pentene-1 and hexene-1 are particularly preferred comonomersfor ethylene copolymerisation.

Typical diluents employed in such alpha-monoolefin polymerizationsinclude hydrocarbons having 3 to 12, preferably 3 to 8, carbon atoms permolecule, such as linear alkanes such as propane, n-butane, n-hexane andn-heptane, or branched alkanes such as isobutane, isopentane, toluene,isooctane and 2,2,-dimethylpropane, or cycloalkanes such as cyclopentaneand cyclohexane or their mixtures. In the case of ethylenepolymerization, the diluent is generally inert with respect to thecatalyst, cocatalyst and polymer produced (such as liquid aliphatic,cycloaliphatic and aromatic hydrocarbons), at a temperature such that atleast 50% (preferably at least 70%) of the polymer formed is insolubletherein. Isobutane is particularly preferred as the suspending mediumfor ethylene polymerisation

The operating conditions can also be such that the monomers (egethylene, propylene) act as the principal suspending medium or diluentas is the case in so called bulk polymerisation processes. The slurryconcentration limits in volume percent have been found to be able to beapplied independently of molecular weight of suspension medium andwhether the suspension medium is inert or reactive, liquid orsupercritical. Propylene monomer is particularly preferred as thediluent for propylene polymerisation.

Methods of molecular weight regulation are known in the art and need notbe described in detail. When using Ziegler-Natta, metallocene andtridentate late transition metal type catalysts, hydrogen is preferablyused, a higher hydrogen pressure resulting in a lower average molecularweight. When using chromium type catalysts, polymerization temperatureis preferably used to regulate molecular weight.

In commercial plants, the particulate polymer is separated from thediluent in a manner such that the diluent is not exposed tocontamination so as to permit recycle of the diluent to thepolymerization zone with minimal if any purification. Separating theparticulate polymer produced using the process of the present inventionfrom the diluent typically can be by any method known in the art forexample it can involve either (i) the use of discontinuous verticalsettling legs such that the flow of slurry across the opening thereofprovides a zone where the polymer particles can settle to some extentfrom the diluent or (ii) continuous product withdrawal via a single ormultiple withdrawal ports, the location of which can be anywhere on theloop reactor but is preferably adjacent to the downstream end of ahorizontal section of the loop. Any continuous withdrawal ports willtypically have an internal diameter in the range 2-25, preferably 4-15,especially 5-10 cm

Use of concentrating devices on the withdrawn polymer slurry, preferablyhydrocylones (single or in the case of multiple hydrocyclones inparallel or series), further enhances the recovery of diluent in anenergy efficient manner since significant pressure reduction andvaporisation of recovered diluent is avoided

It has been found that both the slurry concentration and the minimumacceptable Froude number in the reactor loop can be optimised bycontrolling the average particle size and/or the particle sizedistribution of the powder within the reactor loop. The principaldeterminant of the average particle size of the powder is the residencetime in the reactor. The particle size distribution of the catalyst canbe affected by many factors including the particle size distribution ofthe catalyst fed to the reactor, the initial and average catalystactivity, the robustness of the catalyst support and susceptibility ofthe powder to fragment under reaction conditions. Solids separatingdevices (such as hydrocyclones) can be used on the slurry withdrawn fromthe reactor loop to further assist in control of the average particlesize and the particle size distribution of the powder in the reactor.The location of the withdrawal point for the concentrating device andthe design and operating conditions of the concentrating device system,preferably the at least one hydrocyclone recycle loop, also enables theparticle size and particle size distribution within the reactor to becontrolled. The average particle size is preferable between 100 and 1500microns, most preferably between 250 and 1000 microns.

The withdrawn, and preferably concentrated, polymer slurry isdepressurised, and optionally heated, prior to introduction into aprimary flash vessel. The stream is preferably heated afterdepressurisation.

The diluent and any monomer vapors recovered in the primary flash vesselare typically condensed, preferably without recompression and reused inthe polymerization process. The pressure of the primary flash vessel ispreferably controlled to enable condensation with a readily availablecooling medium (eg cooling water) of essentially all of the flash vapourprior to any recompression, typically such pressure in said primaryflash vessel will be 4-25 for example 10-20, preferably 15-17 bara. Thesolids recovered from the primary flash vessel is preferably passed to asecondary flash vessel to remove residual volatiles. Alternatively theslurry may be passed to a flash vessel of lower pressure than in the theabove mentioned primary vessel such that recompression needed tocondense the recovered diluent. Use of a high pressure flash vessel ispreferred. The process according to the invention can be used to produceresins which exhibit specific density in the range 0.890 to 0.930 Lowdensity), 0.930 to 0.940 (medium density) or 0.940 to 0.970 (highdensity).

The process according to the invention is relevant to all olefinpolymerisation catalyst systems, particularly those chosen from theZiegler-type catalysts, in particular those derived from titanium,zirconium or vanadium and from thermally activated silica or inorganicsupported chromium oxide catalysts and from metallocene-type catalysts,metallocene being a cyclopentadienyl derivative of a transition metal,in particular of titanium or zirconium.

Non-limiting examples of Ziegler-type catalysts are the compoundscomprising a transition metal chosen from groups IIIB, IVB, VB or VIB ofthe periodic table, magnesium and a halogen obtained by mixing amagnesium compound with a compound of the transition metal and ahalogenated compound. The halogen can optionally form an integral partof the magnesium compound or of the transition metal compound.

Metallocene-type catalysts may be metallocenes activated by either analumoxane or by an ionizing agent as described, for example, in PatentApplication EP-500,944-A1 (Mitsui Toatsu Chemicals).

Ziegler-type catalysts are most preferred. Among these, particularexamples include at least one transition metal chosen from groups IIIB,IVB, VB and VIB, magnesium and at least one halogen. Good results areobtained with those comprising: from 10 to 30% by weight of transitionmetal, preferably from 15 to 20% by weight,

from 20 to 60% by weight of halogen, the values from 30 to 50% by weightbeing preferred,

from 0.5 to 20% by weight of magnesium, usually from 1 to 10% by weight,

from 0.1 to 10% by weight of aluminium, generally from 0.5 to 5% byweight,

the balance generally consists of elements arising from the productsused for their manufacture, such as carbon, hydrogen and oxygen. Thetransition metal and the halogen are preferably titanium and chlorine.

Polymerizations, particularly Ziegler catalysed ones, are typicallycarried out in the presence of a cocatalyst. It is possible to use anycocatalyst known in the art, especially compounds comprising at leastone aluminium-carbon chemical bond, such as optionally halogenatedorganoaluminium compounds, which can comprise oxygen or an element fromgroup I of the periodic table, and aluminoxanes. Particular exampleswould be organoaluminium compounds, of trialkylaluminiums such astriethylaluminium, trialkenylaluminiums such as triisopropenylaluminium,aluminium mono- and dialkoxides such as diethylaluminium ethoxide, mono-and dihalogenated alkylaluminiums such as diethylaluminium chloride,alkylaluminium mono- and dihydrides such as dibutylaluminium hydride andorganoaluminium compounds comprising lithium such as LiAl(C₂ H₅)₄.Organoaluminium compounds, especially those which are not halogenated,are well suited. Triethylaluminium and trjisobutylaluminium areespecially advantageous.

The chromium-based catalyst is preferred to comprise a supportedchromium oxide catalyst having a titania-containing support, for examplea composite silica and titania support. A particularly preferredchromium-based catalyst may comprise from 0.5 to 5 wt % chromium,preferably around 1 wt % chromium, such as 0.9 wt % chromium based onthe weight of the chromium-containing catalyst. The support comprises atleast 2 wt % titanium, preferably around 2 to 3 wt % titanium, morepreferably around 2.3 wt % titanium based on the weight of the chromiumcontaining catalyst. The chromium-based catalyst may have a specificsurface area of from 200 to 700 m.sup.2/g, preferably from 400 to 550m.sup.2/g and a volume porosity of greater than 2 cc/g preferably from 2to 3 cc/g.

Silica supported chromium catalysts are typically subjected to aninitial activation step in air at an elevated activation temperature.The activation temperature preferably ranges from 500 to 850.degree. C.,more preferably 600 to 750.degree. C.

The reactor loop is preferably used to-make multi-modal polymers. Themulti-modal polymers being made in a single reactor or in multiplereactors. The reactor loop can comprise one or more loop reactorsconnected in series or in parallel. The reactor loop may also bepreceded or followed by a polymerisation reactor that is not a loopreactor.

In the case of series reactors, the first reactor of the series issupplied with the catalyst and the cocatalyst, and each subsequentreactor is supplied with, at least, ethylene and with the slurry arisingfrom the preceding reactor of the series, this mixture comprising thecatalyst, the cocatalyst and a mixture of the polymers produced in thepreceding reactors of the series. It is optionally possible to supplythe second reactor and/or, if appropriate, at least one of the followingreactors with fresh catalyst and/or cocatalyst. However, it ispreferable to introduce the catalyst and the cocatalyst exclusively intothe first reactor.

In the case where the plant comprises more than two reactors in series,the polymer of highest melt index and the polymer of lowest melt indexcan be produced in two adjacent or non-adjacent reactors in the series.Hydrogen is maintained at (i) a low (or zero) concentration in thereactor(s) manufacturing the high molecular weight components, e.g.hydrogen percentages including between 0-0.1 vol % and at (ii) a veryhigh concentration in the reactor(s) manufacturing the low molecularweight components e.g. hydrogen percentages between 0.5-2.4 vol %. Thereactors can equally be operated to produce essentially the same polymermelt index in successive reactors.

Particular sensitivity to increasing reactor diameters (and associatedcross-sectional compositional, thermal or particulate gradients) hashowever been to related to production of polymer resins where polymer ofeither high or low molecular weight resins has been known to lead toincreased fouling concerns. Particularly when producing polymers ofmolecular weights less than 50 kDaltons or greater than 150 kDa. Theseconcerns have particularly been confirmed to be accentuated at lowpolymer solids concentrations in the reactor loop. When producingpolymers of molecular weights less than 50 kDaltons or greater than 200kDa (or melt index below 0.1 and above 50) in large diameter reactors ithas however surprisingly been discovered that fouling is decreased whensolids loadings are increased to above 20 vol %, particularly above 30vol %.

1. A loop reactor of a continuous tubular construction comprising atleast two horizontal sections, at least two vertical sections and one ormore pumps, said pumps being located on one or more horizontal sectionswherein the average internal diameter of the vertical sections is 5-90%greater than the average internal diameter of the horizontal sections.2. A loop reactor as claimed in claim 1 wherein the average internaldiameter of the loop reactor is over 300 millimeters.
 3. A loop reactoras claimed in claim 1 wherein the average internal diameter of the loopreactor is in the range 600 to 750 millimeters.
 4. A loop reactor asclaimed in claim 1 wherein the average internal diameter of thehorizontal sections is in the range 500-700 millimeters and the averageinternal diameter of the vertical sections is in the range 600-900millimeters.
 5. A loop reactor as claimed in claim 1 wherein the averageinternal diameter of the vertical sections is 5-50% greater than theaverage internal diameter of the horizontal sections.
 6. A loop reactoras claimed in claim 1 wherein the ratio of the reactor length to theaverage internal diameter of the loop reactor is less than
 500. 7. Aloop reactor as claimed in claim 1 wherein the reactor size is over 50m³.
 8. A loop reactor as claimed in claim 1 wherein the horizontalsections of the loop reactor consist of no more than 20% of the totalreactor length.
 9. A loop reactor as claimed in claim 5 wherein theaverage internal diameter of the two vertical sections is 10-30% greaterthan the average internal diameter of the horizontal sections.
 10. Aloop reactor as claimed in claim 6 wherein the ratio of the reactorlength to the average internal diameter of the loop reactor is less than250.
 11. A loop reactor as claimed in claim 7 wherein the reactor sizeis 75-150 m³.